After spending half an hour on the phone with Mobilicity and another hour and a half with Samsung - during which I've had my phone restored to factory settings twice - I was finally told that my MMS had stopped working simply because my Nexus S didn't like Ice Cream Sandwich and it's a known bug that Google is supposedly working on resolving. It sucks because the ICS was an over-the-air update that came from Samsung - they should have tested it to make sure it's compatible before pushing it out.
Meanwhile, my dear friends had somehow all gotten the bug to send me pictures that I keep telling them I can't see rather than through e-mail or whatsapp. I was almost at my wit's end when B came to the rescue and got Gingerbread back on my phone. Unfortunately I lost some contacts and apps during the switch because Gingerbread didn't have the data portability functionality, and the stock camera app in Gingerbread is pathetically laughable. But since I switched back to Gingerbread, my contact list now works with the Facebook app to show the profile picture of that contact rather than having to manually take their pics again. And MMS now works! Hooray!